Research On Servo Drive Technology For BHF Control

CNC technology based on servo drive has been widely used in manufacturingindustry, and has become the core technology of modern manufacturing. Its application informing manufacturing field is going to be an inevitable trend. Due to heavy loads,diversities of processes and other difficulties in metal forming, servo drive technology isstill not widely used in forming manufacturing. By applying the servo drive technology inblank holder force control during deep drawing process, the BHF can be changed alongwith the stroke of arbitrary in order to reach the optimal control goal, according to thecharacteristics of sheet metal forming and the process requirements. This new blankholding force control technology will play an important role in promoting the automation,flexibility and intelligence of the sheet metal forming. It also reflects the advancedforming concept which is more precise, faster and more economical, presenting thedevelopment tendency of the advanced stamping technology.This paper presents a servo drive blank holding force control method whichcomposes a servo motor and a six-bar mechanism acting as the power of the controlsystem and the main part of the mechanical system and a new BHF control system isdeveloped. Researches on the basic principles, optimizations of the mechanical systemand the numerical control system, the designs of the hardware and software of the controlsystem and the related processing theories were made.First, the formula of critical BHF of flange region in axisymmetric deep drawing isdeduced under the plane strain assumption and the energy principle. Meanwhile, thevariation curve of the BHF versus the travel distance changing is drawn. Comparisonsunder the assumptions of plane stress, plane strain and equivalent strain inverselyproportional to radial coordinates in the flange region is made and results show that theplane stress condition is the closest to the actual situation.Then, taking an upright constructive mold for instance, the BHF actuator withdouble-slider and six-bar mechanism is designed adopting the composite servo drivemethod. The analysis gets the motion equation of the six-bar mechanism which can optimize linkage sizes under constraint conditions such as the drive ratio in the ratedworking stroke and the drive angle, and the goal is to optimize the overall size.Simulations of the blank holding system by virtual prototype technology are taken underthe circumstances of the constant BHF and the variable BHF. And the feasibility of thecomposite servo drive method is proved under the variable BHF by adjusting the motorspeed. The analysis indicates that it only requires motors below2kW to generate200kNBHF adopting the new drive method. In addition, how the linkage size machining errorsinfluence the precise movement of blank holders are analyzed. A linkage size toleranceselecting method is proposed as a basis to design blank holding mechanisms.For inverted pattern structures, a mathematical model indicating the input and outputof the BHF actuator is established, adopting the composite drive method, and consideringhow the rigid and flexible distances effect on outcome speeds. In order to minimize thedifference value between the binder speed and the sliding block speed, mechanismparameters are determined based on the BHF-slide distance curve from a deep-drawingprocess, under the conditions of guaranteeing the working stroke and the optimum valueof loading-pass effects. The simulation of blank holding process proves that the outputsmeet the requirements when the input motor speed is given. The input BHF curve and theoutput BHF curve are very close to each other. And the minimum relative error is lessthan4.7%when the BHF is more than100kN. It also indicates that the maximum torqueis36.6N×m when the max BHF is194kN, in other words, small or medium-sized powermotors can meet the requirements. The analysis results show that the variable BHFcontrol adopting composite method combining the six-bar linkage drive technique andthe servo motor drive technique is feasible.Furthermore, according to the characteristics of numerical system structures and therequirements of the BHF control, the hardware of the numerical BHF control system isdesigned. The system mainly consist servo motor and motion card. And the relatedsoftware is developed according to the requirements of the control system. Models of theservo drive system and the mechanical system are built and simulations are carried out byusing Simulink. It’s shown that the system has good feasibility and good dynamicproperty. Finally, the mechanical system and the numerical control system under the controlof the servo drive BHF are designed realizing the constant BHF and the variable BHFloading for upright constructive molds. The experimental results indicate thattechnological BHF requirements are reached adopting the BHF control method of theservo drive. Torque and power of motors are lower than rated values under thecircumstance that the blank holding force is up to about200kN.